Holocene glacier fluctuations and tephrochronology of the Öræfi district, Iceland
Gudmundsson, Hjalti Johannes
The aims of this thesis are to refine the tephrochronology of the Öræfi district, SE Iceland and assess the Holocene glacier fluctuations of the Öræfajökull ice cap. The pattern and timing of glacier fluctuations are determined using glacial geomorphology and tephrochronology, and the implications for palaeoclimate are assessed. Iceland is important to the study of global and regional climatic change because it is located close to both the marine and atmospheric Polar Fronts widely regarded as the key factors in the climate of the North Atlantic region. Six outlet glaciers were studied: Svinafellsjökull, Virkisjökull, Kotárjökull and Kviárjökull originating from the Öræfajökull ice cap and Skaftafellsjökull and Morsárjökull originating from the Vatnajökull ice cap. A long history of glacier fluctuations were found with a similar temporal pattern of glacier oscillation between the outlets of Vatnajökull and Öræfajökull. A maximum of eight advances have been identified. The oldest advance is inferred to date from the maximum of the last Glaciation ca. 18000 yrs BP. The first advance in the Holocene occurred ca. 9700 BP during a still-stand of the last Termination. The onset of the Neoglaciation occurred between 6000 BP and 4600 BP with an expansion of all of the studied glaciers. Subsequent smaller advances have been dated to ca. 3200 BP, ca. 1800 BP, ca. 700 BP, ca. 200 BP and ca. 80 BP. The most significant movement of the Polar front during the Holocene is likely to have occurred around 5000 BP, and, as a consequence, an estimated temperature cooling of ca. 2.5°C took place in Iceland, perhaps the greatest cooling since the last Termination. Within the broad pattern of change, glaciers in the study area show variability which represents local precipitation patterns, contrasting topography and change in glacier process. In this thesis a total of 22 silicic tephra layers are identified from over 90 profiles in the study area. The majority of these layers are dated to the latter part of the Holocene. Three silicic tephras were deposited during historical time (post 900 AD) namely, Vö ca. 900AD,HI104 and Ö1362. The Vö ca. 900 AD and the H1104 tephras are located for the first time. Specific prehistoric (pre 900 AD) tephras identified include Hekla-Ö, Hekla-4 and Hekla-S. The tephrochronology of the Öræfi district is also used to assess the eruption history of the Öræfajökull stratovolcano during the Holocene. Prehistoric eruptions are dated to ca. 9200 BP, ca. 6500 BP(?), ca. 4700 BP, ca. 2800 BP and ca. 1500 BP. Jökulhlaups accompanied the eruptions of 1727 AD, 1362 AD and ca. 1500 BP and are likely to have followed older eruptions of the volcano. A strong relationship occurs between volcanic activity of the Öræfajökull stratovolcano and the pattern of glacier fluctuations. This is explained as a response to isostatic crustal adjustment during ice cap growth and decay, and indicates a general relationship between volcanic activity and climate change.